Effects of Annealing Time and Temperature on Hydrogen in Doped and Intrinsic Amorphous Silicon

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EFFECTS OF ANNEALING TIME AND TEMPERATURE ON HYDROGEN IN DOPED AND INTRINSIC AMORPHOUS SILICON S. E. READY AND J. B. BOYCE XEROX Palo Alto Research Center, 3333 Coyote Hill Road, Palo Alto, CA 94304. ABSTRACT The local environment and diffusion of hydrogen in hydrogenated amorphous silicon (a-Si:H) depend on both temperature and doping. Previous studies of hydrogen evolution indicate that the manner in which hydrogen diffuses and desorbs from the material depends on doping and may occur at relatively low temperatures over extended time frames. We have examined the microstructure of hydrog.en in intrinsic, boron doped and phosphorous doped a-Si:H as a function of annealing temperature and annealing time using hydrogen NMR. Changes in both the H NMR spectrum and spin-lattice relaxation times occur. We find annealing time has only a small effect on these parameters, whereas the annealing temperature has a substantial effect. The bonded-H content drops and the molecular-H2 content is seen to decrease slightly as the samples are annealed to higher temperatures. However the bonded-H remains essentially constant for long time, low-temperature anneals, while the molecular-H2 content is also seen to diminish slightly. The changes are more profound for B-doped samples than for P-doped or intrinsic material, consistent with the conclusions of other studies. INTRODUCTION While hydrogen effectively reduces the number of electronic defects in amorphous silicon, thereby making electronic devices possible, it also introduces unique stability problems. Hydrogenated amorphous silicon can be regarded as existing in a relatively soft metastable equilibrium in which the population of electronic defects are sensitive to a number of perturbations. Central to most proposed mechanisms of persistent defect creation and subsequent removal is microscopic structural changes mediated by hydrogen's ability to diffuse through the lattice [1-3]. There is evidence that hydrogen may exist as a glass-like sub-matrix which, when raised to elevated temperatures, rapidly reaches thermal equilibrium [3]. This equilibrium can subsequently be frozen in when cooled rapidly. The hydrogen glass characteristic equilibrium temperature has been shown to be different for different types of doping (130 0C for n-type and 90 0C for p-type). Similarly, hydrogen effusion data has shown [1] that, while hydrogen evolves at about 350 0 C for n-type and intrinsic material, p-type material exhibits an evolution peak at a lower temperature of about 200 0C. Also, recent investigations into the effects of Iong time anneals at temperatures below the deposition temperature in p-type material suggest that the hydrogen redistributes to gradually form non-bonded H2 prior to evolving. Nuclear magnetic resonance (NMR) is quite suited for investigating the state of the hydrogen in amorphous silicon since it can selectively examine the local environment of distinct atomic species. NMR investigations have yielded much information on the local atomic bonding and microstructure of a-Si:H